Method for processing compositions containing 1,1,2,2-tetramethoxyethane and glyoxal dimethyl acetal

ABSTRACT

A process is proposed for distillatively working up an aqueous composition which comprises 1,1,2.2-tetramethoxyethane, glyoxal dimethyl acetal and methanol, wherein the workup of this composition is carried out in a dividing wall column to form low boiler, medium boiler and/or high boiler fractions, and a dividing wall is disposed in the longitudinal direction in the dividing wall column to form an upper common column region, a lower common column region, a feed section having rectifying section and stripping section and a withdrawal section having stripping section and rectifying section, and the aqueous composition is fed into the middle region of the feed section.

This application is a National Stage of PCT/EP2004/009832 filed Sep. 3,2004 which in turn claims priority from German Application 103 40 738.3,filed Sep. 4, 2003.

DESCRIPTION

The present invention relates to a process for distillatively working upan aqueous composition which comprises 1,1,2,2-tetramethoxyethane,glyoxal dimethyl acetal and methanol. These aqueous compositions aregenerally obtained as the reaction effluent in processes for preparingmono- or diacetals of glyoxal.

Monoacetals (e.g. glyoxal dimethyl acetal) and diacetals of glyoxal(e.g. 1,1,2,2-tetra-methoxyethane) are important intermediates inorganic synthesis.

It is common knowledge that mono- or diacetals of glyoxal can beprepared in an equilibrium reaction by acid-catalyzed acetalization ofglyoxal with monohydric alcohols R—OH: the acid-catalyzed acetalizationof glyoxal with monohydric alcohols is a complex reaction in which notonly the monoacetal and the diacetal, but also a multitude of oligomersand/or cyclic by-products may be formed (see, for example, J. M.Kliegmann et al. in J. Org. Chem., Vol. 38 (1973), p. 556).

The reaction of glyoxal with water-miscible alcohols, as described, forexample, in U.S. Pat. No. 2,360,959, usually affords low yields ofdiacetal. In order to increase the yield of diacetal, water has to beremoved continuously from the reaction mixture in order to appropriatelyshift the equilibrium of the reaction. This removal is difficult toperform, especially in the case of water-miscible alcohols. For thispurpose, GB 359 362 recommends the use of an inert solvent as anazeotroping agent for removing water from the reaction mixture. Theselection of a suitable azeotroping agent is based on its boiling pointand its boiling behavior in the reaction mixture. However, the use ofazeotroping agents leads to additional costs.

In Synth. Comm. 1988, 18, pages 1343 to 1348, Chastrette et al. describethe acetalization of glyoxal in chloroform. The catalyst used in orderto obtain high yields of diacetal in a prolonged reaction time iszirconium sulfate. In addition, the solvent and azeotroping agent usedis chloroform which is damaging to health, but leads to the additionalcosts which have already been mentioned.

In order to obtain glyoxal mono- or diacetals in good yields from thereaction effluent of the reaction of glyoxal with alcohols, the priorart describes multistage and complex separating processes.

EP 0 607 722 describes a process in which the distillative workup iscarried out in at least 5 columns and additionally at different pressurelevels, in order to obtain an aqueous glyoxal dimethyl acetal solution.In a first column, the majority of the excess alcohol is removed atatmospheric pressure (step 1). Subsequently, an aqueous mixture whichcomprises the majority of the glyoxal acetals is prepared in a secondcolumn (step 2). After distillative isolation in a third column, thediacetal is obtained from this mixture as an aqueous azeotrope (step 3)and fed to an additional reactor in which it is dissociated back toglyoxal and methanol. After the alcohol has been removed from thissolution in a fourth column (step 5), the glyoxal is fed back to thereactor after the glyoxal concentration has been increased to 70% in afurther fifth column (step 6). The bottom product obtained during thisdistillation comprises the monoacetal and can be concentrated to thedesired concentration in a further apparatus (step 4).

EP 0 847 976 A1 describes a multistage distillative workup of a similareffluent in which the excess alcohol is first removed (step 1), then thediacetal is obtained with the addition of water as an aqueoushomoazeotrope (step 2) and this is isolated by an azeotroping agentdistillation (step 5) and recycled into the process. The monoacetal issubsequently obtained by a steam distillation and a subsequentfractional distillation (steps 3 and 4) and the resulting distillationresidues are recycled.

In addition, it is common to the two latter processes that the reactantsused are concentrated glyoxal solutions, since commercial glyoxalsolutions only have a glyoxal content of about 40% and their use resultsin lower yields being achieved. Methods for concentrating this solutionare known and are described, for example, in EP 1 300 383 A2. However,additional apparatus is required to concentrate the commercial glyoxalsolutions.

The prior art processes thus imply complex plants, internals whoseacquisition has high capital costs and a high energy requirement.

It is an object of the present invention to provide a process forworking up aqueous compositions which comprise1,1,2,2-tetramethoxyethane, glyoxal dimethyl acetal and methanol, whichcan afford the corresponding pure products by suitable processconfiguration with apparatus of low complexity.

We have found that this object is achieved by a process fordistillatively working up an aqueous composition which comprises1,1,2,2-tetramethoxyethane, glyoxal dimethyl acetal and methanol.

In the process according to the invention, the composition is worked upto form at least one low boiler fraction, at least one medium boilerfraction and at least one high boiler fraction in a dividing wall columnin which a dividing wall is arranged in the longitudinal direction ofthe column to form an upper common column region, a lower common columnregion, a feed section having rectifying section and stripping sectionand a withdrawal section having rectifying section and strippingsection, the dividing wall is arranged between the upper and the lowercommon column region and the aqueous composition is fed to the middleregion of the feed section, and at least one medium boiler fractioncomprising aqueous 1,1,2,2-tetramethoxyethane is obtained as asidestream from the middle region of the withdrawal section, at leastone high boiler fraction comprising glyoxal dimethyl acetal from thelower common column region and at least one low boiler fractioncomprising methanol from the upper common column region.

Aqueous Composition

The aqueous composition employed in the process according to theinvention preferably has one or more of the following contents whose sumdoes not exceed 100% by weight:

-   -   from 8 to 28% by weight of 1,1,2,2-tetramethoxyethane,    -   from 2 to 12% by weight of glyoxal dimethyl acetal,    -   from 40 to 80% by weight of methanol and    -   from 5 to 18% by weight of water.

When 1,1,2,2-tetramethoxyethane is to be obtained from the aqueouscomposition by the process according to the invention, the aqueouscomposition employed in the process according to the inventionpreferably has one or more of the following contents whose sum does notexceed 100% by weight:

-   -   from 12 to 16% by weight of 1,1,2,2-tetramethoxyethane,    -   from 2 to 8% by weight of glyoxal dimethyl acetal,    -   from 60 to 80% by weight of methanol and    -   from 5 to 12% by weight of water.

In a preferred embodiment of the present invention, the aqueouscomposition which is employed in the process according to the inventionadditionally contains from 0 to 4% by weight of glyoxal.

The aqueous composition employed in the process according to theinvention may additionally comprise further compounds which are selectedfrom the group consisting of 2,3-dimethoxy-1,4-dioxane and2-(dimethoxymethyl)-1,3-dioxolane.

When 2,3-dimethoxy-1,4-dioxane is present in the aqueous composition,its content in the aqueous composition is preferably from 0 to 10% byweight, more preferably from 0 to 5% by weight. When2-(dimethoxymethyl)-1,3-dioxolane is present in the aqueous composition,its content in the aqueous composition is preferably from 0 to 8% byweight, more preferably from 0 to 4% by weight.

In a preferred embodiment, the aqueous composition which is used in theprocess according to the invention is prepared by processes which aredescribed in EP 1 300 383 A2 or EP 0 847 976 A1, whose disclosurecontents are fully incorporated by reference into the present patentapplication.

The process of EP 1 300 383 A2 serves to prepare diacetals of glyoxal.The process comprises the reaction of from 40 to 75% by weight aqueousglyoxal with methanol in the presence of an acidic catalyst. Thisinvolves contacting a liquid mixture which, at the start of thereaction, contains methanol and glyoxal in a molar ratio of at least15:1 and water in a concentration of not more than 8% by weight with theacidic catalyst until the concentration of the1,1,2,2-tetramethoxyethane formed In the reaction mixture has reached atleast 70% of the equilibrium concentration. Not more than 5% by weightof the methanol is distilled off at the same time or beforehand. Thereaction effluents resulting from this process comprise at least1,1,2,2-tetramethoxyethane, glyoxal dimethyl acetal, water and methanol.Before the reaction, the aqueous glyoxal is preferably brought bydewatering to a content of from 60 to 75% by weight of glyoxal. Thisresults in better yields being obtained in the acetalization. Thedewatering is preferably effected under a reduced pressure of from 100to 200 mbar.

In a further preferred embodiment, the aqueous composition which isemployed in the process according to the invention is obtained in aprocess according to EP 0 847 976 A1. In this process, glyoxalmonoacetals of the general formula (I)

are prepared by reacting a mixture of glyoxal and glyoxal bisacetals ofthe general formula (II)

in the presence of an acidic catalyst with an excess of a monohydricalcohol R—OH until the reaction equilibrium has been attained. Theglyoxal solution used in this preparation is preferably in the form ofan aqueous solution, and it is appropriate to use the customarytechnical aqueous solutions having a glyoxal content of from 20 to 60%by weight, preferably from 30 to 50% by weight. However, before it isreacted, the aqueous glyoxal solution may also be brought by dewateringto a content of from 60 to 75% by weight, preferably from 65 to 70% byweight.

The R radical of the formulae I and II is derived directly from thealcohol of the general formula ROH used and is thus the same. R isbranched or unbranched C₁–C₄–alkyl and/or branched or unbranchedC₂–C₄–alkenyl. C₁–C₄–Alkyl is, for example, methyl, ethyl, propyl,isopropyl and butyl. C₂–C₄–Alkenyl is, for example, vinyl, propenyl andisopropenyl. Particularly preferred R radicals are methyl and ethyl.

Dividing Wall Column

For the continuous distillative separation of multisubstance mixtures,various process variants can be used. In the simplest case, the feedmixture is separated into two fractions, a low-boiling top fraction anda high-boiling bottom fraction. In the separation of feed mixtures intomore than two fractions, a plurality of distillation columns has to beused in this process variant. In order to reduce the apparatus demands,columns having liquid or vaporous sidestreams are used if possible inthe separation of multisubstance mixtures. However, the possibility ofemploying distillation columns having sidestreams is highly restrictedin that the products removed at the sidestream takeoff points are nevercompletely pure. In the case of side withdrawals in the rectifyingsection, which are typically in liquid form, the side product stillcontains fractions of low-boiling components which are to be removed viathe top. The same applies for side withdrawals in the stripping sectionwhich are usually in vaporous form and in which the side product stillcontains high boiler fractions. The use of conventional sidestreamcolumns is therefore restricted to cases in which contaminated sideproducts are permissible.

A means of remedy is offered by what are known as dividing wall columnswhich are described, for example, in EP-A 0 122 367. Dividing wallcolumns are distillation columns having vertical dividing walls whichprevent transverse mixing of liquid and vapor streams in subregions. Thedividing wall which preferably consists of a metal sheet divides thecolumn in the longitudinal direction in its middle region into a feedsection and into a withdrawal section.

The dividing wall column used in the process according to the inventionthus has a dividing wall which is aligned in the longitudinal directionof the column and divides the column interior into the followingsubregions: an upper common column region, a lower common column regionand a feed section and a withdrawal section, each having rectifyingsection and stripping section. The mixture to be separated is introducedin the region of the feed section, and at least one high boiler fractionis withdrawn from the column bottom, at least one low boiler fractionvia the column top and a medium boiler fraction from the region of thewithdrawal section. With regard to the arrangement of the regions in thedividing wall column, reference is made to DE 101 00 552 A1, whosedisclosure content is hereby incorporated by reference into the presentpatent application.

In the dividing wall column used in the process according to theinvention, it is preferred that

-   -   the upper common subregion has from 5 to 50%, preferably from 15        to 30%, of the total number of theoretical plates of the        dividing wall column,    -   the rectifying section of the feed section has from 5 to 50%,        preferably from 15 to 30%, of the total number of theoretical        plates of the dividing wall column,    -   the stripping section of the feed section has from 5 to 50%,        preferably from 15 to 30%, of the total number of theoretical        plates of the dividing wall column,    -   the stripping section of the withdrawal section has from 5 to        50%, preferably from 15 to 30%, of the total number of        theoretical plates of the dividing wall column,    -   the rectifying section of the withdrawal section has from 5 to        50%, preferably from 15 to 30%, of the total number of        theoretical plates of the dividing wall column, and    -   the lower common section has from 5 to 50%, preferably from 15        to 30%, of the total number of theoretical plates of the        dividing wall column,        the total number of theoretical dividing wall plates of the        dividing wall column being 100%. It is thus preferably ensured        that the theoretical plates of the dividing wall column are        divided between the individual column regions in such a way that        each has from 5 to 50% of the total number of theoretical plates        of the dividing wall column. The feed section and the withdrawal        section form the middle region of the dividing wall column.

The dividing wall column used in the process according to the inventionpreferably has from 30 to 120, more preferably from 50 to 100, mostpreferably from 60 to 80, theoretical plates.

The operating pressure of the dividing wall column in the processaccording to the invention is preferably from 300 to 1500 mbar, morepreferably from 400 to 600 mbar.

In the dividing wall column used in the process according to theinvention, the sum of the number of theoretical plates in the strippingand rectifying section in the feed section is preferably from 80 to110%, more preferably from 90 to 100%, of the sum of the number ofplates in the stripping and rectifying section in the withdrawalsection.

The dividing wall column may be equipped in the stripping and/orrectifying sections of the feed and/or withdrawal section or in partsthereof with structured packings or random packings. In addition, it ispossible that the dividing wall is configured with heat insulation inthese subregions.

It is also possible to configure the dividing wall in the form ofloosely inserted and adequately sealed subsegments. In this case, thedividing wall is not welded into the column. This leads to a furtherreduction in costs in the production and construction of dividing wallcolumns.

Particularly advantageously, the loose dividing wall may have internalmanholes or removable segments which allow access within the dividingwall column from one side of the dividing wall to the other side.

With regard to the separating internals which can be used in thedividing wall column, there are in principle no restrictions: bothrandom packings and structured packings or trays are suitable for thispurpose. For reasons of cost, trays, preferably valve or sieve trays,are generally used in columns having a diameter above 1.2 meters. Whenthe aqueous composition is worked up, it is recommended, especially inthe dividing wall region and in the common upper column region, to usestructured packings as internals. In this case, structured sheet metalor fabric packings having a specific surface area of preferably from 100to 1000 m²/m³, more preferably from about 250 to 500 m²/m³, areparticularly suitable. In the lower common subregion of the dividingwall column, trays may also be used with preference, more preferablyvalve trays.

In the event of particularly high requirements on the product purity, itis favorable, especially in the case that structured packings are usedas separating internals, to provide the dividing wall with thermalinsulation. Such a configuration of the dividing wall is described, forexample, in EP 0 640 367. A double-walled configuration with a narrowgas space in between is particularly favorable.

The position of the dividing wall in the individual subregions of thedividing wall column is preferably adjusted in such a way that the crosssections of feed and withdrawal section have different surface areas.

In the process according to the invention, the aqueous composition, as afeed stream to the dividing wall column, is preferably partly or fullypre-evaporated in a pre-evaporator and fed to the column in biphasicform or in the form of a gaseous and of a liquid stream.

In this case, the feed point and the sidestream takeoff point of thedividing wall column, with respect to the position of the theoreticaldividing wall plates, are preferably disposed at different heights inthe column, so that the feed point is disposed from 1 to 20, morepreferably from 5 to 10, theoretical plates higher or lower than thesidestream takeoff point. However, the feed point and the sidestreamtakeoff point may also be at the same height.

The liquid distribution in the individual subregions of the column ispreferably deliberately nonuniform. In such a process variant, theliquid distribution in the individual subregions of the dividing wallcolumn can preferably in each case be adjusted separately. This allowsthe total energy amount which is required to separate the aqueouscomposition to be minimized.

Particularly advantageously, the liquid may be introduced to anincreased extent in the wall region in the rectifying sections of thedividing wall column and to a reduced extent in the wall region in thestripping sections of the dividing wall column. This prevents undesiredcreep streams and increases the achievable end product purities.

In addition to a top and a bottom product, side products may likewise beobtained in pure form from dividing wall columns. When multisubstancemixtures are separated into a low boiler, a medium boiler and a highboiler fraction, specifications of the maximum permissible level of lowand high boilers in the medium boiler fraction are customarily made. Inthis context, specifications are made for components which are criticalto the separation problem, known as key components. These may beindividual key components or the sum of a variety of key components. Inthe present process, methanol (low boilers) and glyoxal dimethyl acetaland 2,3-dimethoxy-1,4-dioxane (high boilers), when present in the columnfeed, are key components.

In a preferred process variant, it is ensured that the abovementionedspecifications with regard to the key components are complied with bycontrolling the division ratio of the liquid at the upper end of thedividing wall and also the heating output of the evaporator in a definedmanner. The division ratio of the liquid at the upper end of thedividing wall is adjusted in such a way that the proportion ofhigh-boiling key components in the liquid reflux via the strippingsection of the withdrawal section is from 10 to 80%, preferably from 30to 50%, of the limiting value permissible in the medium boiler fraction.The heating output in the bottom evaporator of the dividing wall columnis preferably adjusted in such a way that the concentration oflow-boiling key components in the liquid at the lower end of thedividing wall is from 10 to 80%, preferably from 30 to 50%, of thelimiting value permissible in the medium boiler stream. In accordancewith this control, the liquid division at the upper end of the dividingwall is adjusted to the effect that when the contents of high-boilingkey components are higher, more liquid is passed to the feed section,and when the contents thereof are lower, less liquid is passed to thefeed section.

The heating output in the evaporator is preferably adjusted in such away that the concentration of those components in the low boilerfraction for which a certain limiting value for the concentration is tobe attained in the sidestream (key components) is adjusted at the lowerend of the dividing wall in such a way that the concentration ofcomponents of the low boiler fraction in the liquid at the lower end ofthe dividing wall makes up from 10 to 80%, preferably from 30 to 50%, ofthe value which is to be attained in the sidestream product, and theheating output is adjusted to the effect that when the content ofcomponents of the low boiler fraction is higher, the heating output isincreased, and when the content of components of the low boiler fractionis lower, the heating output is reduced.

To compensate for disruptions in the feed rate or the feedconcentration, it is additionally found to be advantageous to useappropriate control methods in the process control system to ensure thatthe flow rates of the liquids which are introduced to the rectifyingsections can never fall below 30% of their normal value.

In a preferred process variant, the vapor stream at the lower end of thedividing wall may be adjusted in such a way that the ratio of vaporstream in the feed section to the vapor stream in the withdrawal sectionis from 0.5 to 2, preferably from 0.9 to 1.1. This is preferablyeffected by the selection and/or dimensioning of separating internalsand/or the incorporation of internals which cause a pressure drop, forexample of perforated plates.

Suitable for withdrawing and dividing the liquids at the upper end ofthe dividing wall and at the side withdrawal point are collectionchambers for the liquid which are mounted either inside or outside thecolumn and assume the function of a pump reservoir or ensure asufficiently high static liquid head, which enable controlled furtherflow of liquid using control units, for example valves. When packedcolumns are used, preference is given to initially capturing the liquidin collectors and passing it from there into an internal or externalcollecting chamber.

In a further embodiment of the present invention, the liquid leaving theupper common section of the column is collected in a collecting chamberdisposed in the column or outside the column and separated at the upperend of the dividing wall in a controlled manner by a fixed setting orclosed-loop control in such a way that the ratio of liquid stream in thefeed section to that of the withdrawal section is from 0.1 to 1.0,preferably from 0.2 to 0.5.

In this case, the liquid is conveyed to the feed section, preferablyusing a pump, or introduced with flow rate control using a static feedhead of at least one meter. This is preferably effected by cascadeclosed-loop control in conjunction with the closed-loop liquid levelcontrol in the collecting chamber.

The liquid leaving the stripping section in the withdrawal section ofthe column is preferably divided between the sidestream and therectifying section of the column by closed-loop control, in such a waythat the amount of liquid introduced to the rectifying section cannotfall below 30% of the normal value.

Preference is given to obtaining at least one high boiler fraction asliquid sidestream in the lower section of the column, more preferablyfrom 1 to 5 theoretical plates above the column bottom. This high boilerfraction comprises glyoxal dimethyl acetal which, for the preferredpreparation of 1,1,2,2-tetramethoxyethane, may optionally be recycledinto the preparation of the aqueous composition which is preferablyeffected in accordance with EP 0 847 976 A1 or EP 1 300 383 A2.

In a preferred embodiment of the process according to the invention, thehigh boiler fraction may thus be reused at least partly to prepare theaqueous composition. Alternatively, it is also possible, before therecycling, to further work up the high boiler fraction which stillcontains a relatively large proportion of glyoxal dimethyl acetal, inorder to obtain pure, aqueous glyoxal dimethyl acetal solution. Thismakes it possible to prepare glyoxal dimethyl acetal.

Preference is further given to recycling the column bottoms, i.e. notthe output from 1 to 5 theoretical plates above the column bottom, intothe bottom evaporator.

The bottom product is preferably withdrawn under closed-loop temperaturecontrol, and the control temperature used is measured at a point in thelower common subregion of the column. This measurement point ispreferably disposed from 3 to 8, more preferably from 4 to 6,theoretical plates above the lower end of the column. The withdrawal ofthe bottom product under closed-loop temperature control is a standardclosed-loop control method for columns which is known to those skilledin the art: when the temperature goes below a certain temperature in thebottoms, less bottom product is removed, which causes the fill level torise and more side product to be removed. Consequently, the mediumboilers move upward from the bottoms.

Preference is given to obtaining glyoxal dimethyl acetal from the highboiler takeoff of the dividing wall column.

The medium boiler fraction is preferably withdrawn in liquid form at thesidestream takeoff point. However, it is additionally also possible thatthe medium boiler fraction is removed in gaseous form at the sidestreamtakeoff point.

The medium boiler fraction is preferably withdrawn at the sidestreamtakeoff point under closed-loop level control, and the control parameterused is the liquid level in the column bottom. In the process accordingto the invention, the medium boiler fraction comprises, in addition towater, 1,1,2,2-tetramethoxyethane. If glyoxal dimethyl acetal is to beobtained by the process according to the invention, this medium boilerfraction may preferably be continuously contacted with an acidic ionexchanger to dissociate the 1,1,2,2-tetramethoxyethane and recycled intothe dividing wall column, preferably into the evaporator which isoptionally connected upstream, or else used for the reactions accordingto EP 0 847 976 A1 or EP 1 300 383 A2, for example by feeding into thedewaterings upstream of the reactions.

The distillate is also preferably withdrawn under closed-looptemperature control, and the control temperature used is preferablymeasured at a point in the upper subregion of the column which isdisposed from 3 to 10, more preferably from 4 to 6, theoretical platesbelow the upper end of the column. The distillate comprises methanol.

In a preferred embodiment of the process according to the invention, themethanolic distillate obtained at the top of the dividing wall column iseither recycled into the dividing wall column or reused to prepare theaqueous composition.

At the upper and at the lower end of the dividing wall column, thedividing wall column used in the process according to the inventionpreferably has sample-taking means, through which liquid and/or gaseoussamples can be withdrawal from the column, continuously or at timeintervals, and be investigated with regard to their composition,preferably by gas chromatography.

The present invention further relates to the use of dividing wallcolumns to distillatively work up an aqueous composition which comprises1,1,2,2-tetramethoxyethane, glyoxal dimethyl acetal and methanol.

Two Thermally Coupled Columns

According to the invention, it is also possible to use thermally coupledcolumns instead of the dividing Wall columns. Arrangements havingthermally coupled columns are equivalent to a dividing wall column withregard to the energy demand. This inventive variant is relevantespecially when existing columns are available, since this avoids newhigh capital costs. The suitable forms of the arrangement may beselected depending on the number of plates of the columns present.

The present invention thus further provides a process for distillativelyworking up an aqueous composition which comprises1,1,2,2-tetramethoxyethane, glyoxal dimethyl acetal and methanol, whichcomprises carrying out the workup of the composition in a combination oftwo distillation columns in the form of a thermal coupling.

The process features which have been listed above for the dividing wallcolumn also apply correspondingly to the embodiment having two thermallycoupled columns. This also applies to the aqueous composition used inthe process according to the invention.

The thermally coupled columns may each be provided with a dedicatedevaporator and/or condenser. In a preferred process variant, onlyliquids are conveyed in the connecting streams between the two thermallycoupled columns. This is particularly advantageous when the thermallycoupled columns are operated with different pressures.

In a preferred connection of the thermally coupled columns, the lowboiler fraction and the high boiler fraction are withdrawn fromdifferent columns, and the operating pressure of the column from whichthe high boiler fraction is withdrawn is set lower than the operatingpressure of the column from which the low boiler fraction is withdrawn,preferably by from 0.5 to 1 bar.

The first column is preferably operated at a pressure of from 500 to1500 mbar, more preferably from 800 to 1200 mbar. The temperature in thefirst column is preferably from 45 to 115° C., more preferably from 55to 105° C. The second column is preferably operated at a pressure offrom 50 to 500 mbar, more preferably from 100 to 200 mbar. Thetemperature in the second column is preferably from 30 to 105° C., morepreferably from 45 to 95° C.

In a particular form of connection, it is possible to partly or fullyevaporate the bottom stream of the first column in an evaporator andsubsequently to feed it to the second column in biphasic form or in theform of a gaseous or of a liquid stream. In addition, it may beadvantageous to subject the feed stream to a pre-evaporation andsubsequently to feed it to the column in biphasic form or in the form oftwo streams. This pre-evaporation is particularly relevant when the feedstream contains relatively large amounts of low boilers. Thepre-evaporation also allows the stripping section of the column to besignificantly deburdened. The same also applies for the abovementionedpre-evaporation in dividing wall columns.

In the variant having thermally coupled columns, the sample-takingmeans, preferably similarly to the embodiment with the dividing wallcolumn, are disposed in the connecting lines between the regions of thethermally coupled columns corresponding to the subregions of thedividing wall column.

The present invention further provides the use of two thermally coupledcolumns for distillatively working up an aqueous composition whichcomprises 1,1,2,2-tetramethoxyethane, glyoxal dimethyl acetal andmethanol. With regard to continuing procedure, reference is made to theprocess outlined above.

Dividing Wall Column or Two Thermally Coupled Columns

In the process according to the invention, preference is given tofeeding water into the feed of the dividing wall column or of the secondthermally coupled column, so that a diacetal concentration in the bottomof the dividing wall column or of the second thermally coupled column ofless than 5% by weight, preferably less than 1% by weight, isestablished. This is advantageous in order to achieve substantiallycomplete diacetal removal. It is useful in this context that, asdescribed in EP 0 847 976 A1, 1,1,2,2-tetramethoxyethane and water forma homoazeotrope. In a further preferred embodiment, the water is fedinto the bottom evaporator feed of the dividing wall column or of thesecond thermally coupled column. This added water may preferably beremoved at least partly from the water stream which is obtained in theevaporation of the glyoxal which preferably takes place before thereaction according to EP 1 300 383 A2 or EP 0 847 976 A1. Alternatively,the water obtained in the purifying distillation of the1,1,2,2-tetramethoxyethane which is downstream of the dividing wallcolumn or of the two thermally coupled columns may also be used.

Furthermore, in a preferred embodiment of the process according to theinvention, commercial, about 40% glyoxal solution is fed into the feedof the dividing wall column or of the second thermally coupled column.This may appropriately be effected via the bottom evaporator feed.

Both in the embodiment with the dividing wall column and in theembodiment with the thermally coupled columns, the bottom evaporator ispreferably a thin-film apparatus, preferably a falling-film evaporator.

The liquid or gaseous aqueous side effluent which is obtainable from thewithdrawal section of the dividing wall column or from the side effluentof the second thermally coupled column and comprises1,1,2,2-tetramethoxyethane is subsequently preferably fed into a furthercolumn which is equipped with trays, structured packings or randompackings, in which the water is removed with the aid of an azeotropingagent distillation. This column preferably has from 10 to 40, morepreferably from 10 to 25, theoretical plates.

The azeotroping agent used is preferably selected from C₅ to C₁₀hydrocarbons. The 1,1,2,2-tetramethoxyethane is obtained from thissecond column preferably at least partly as a liquid or gaseoussidestream in the lower section of the column, preferably from 1 to 5theoretical plates above the column bottom. This distillation takesplace at pressures of preferably from 300 to 1500 mbar, more preferablyfrom 400 to 600 mbar. In a preferred embodiment of the presentinvention, the aqueous side effluent of the dividing wall column or ofthe second thermally coupled column is introduced substantially ingaseous form into the azeotroping agent distillation column, which is aparticularly energy-saving procedure. In this case, the operatingpressure selected for the azeotroping agent distillation column will belower than that of the dividing wall column or of the second thermallycoupled column, preferably by from 10 to 50 mbar. Suitable control unitswhich are known to those skilled in the art will preferably beincorporated into the connecting line between the columns, for example aflap and a diaphragm having appropriate differential pressuremeasurement, which allow the above-described closed-loop control of thesidestream takeoff rate.

The bottom effluent of this azeotroping agent column is preferably fedto the feed point of the dividing wall column. This may be effected, forexample, by feeding into the reactor effluent or into the preheated orsemievaporated column feed.

The 1,1,2,2-tetramethoxyethane obtained in this way has a water contentof preferably less than 1%, more preferably less than 0.1%.

In a further particular embodiment of the present invention, thewastewater obtained at the top of this further column is contactedcontinuously with an acidic ion exchanger. The residence time on the ionexchanger is preferably from 1 to 4 hours at a temperature of from 55 to100° C., preferably from 55 to 80° C.

The invention is illustrated in detail hereinbelow with the aid of thedrawings and also of an inventive example.

FIG. 1 shows a schematic of a dividing wall column (TK) with dividingwall (T) which is disposed vertically therein and divides the columninto an upper common column section 1, into a lower common columnsection 6, a feed section (2, 4) having rectifying section 2 andstripping section 4, and also a withdrawal section (3, 5) with strippingsection 3 and rectifying section 5. The mixture to be separated (A B C)is fed in the middle region of the feed section (2, 4). At the top ofthe column, the low boiler fraction (A) is removed, the high boilerfraction (C) is removed from the column bottom and the medium boilerfraction (B) is removed from the middle region of the withdrawal section(3, 5).

FIG. 2 shows the schematic illustration of a plant for distillativelyworking up the aqueous composition for preparing1,1,2,2-tetramethoxyethane and/or glyoxal dimethyl acetal in a dividingwall column.

The separation of the reaction effluent d into methanolic low boilerfractions i and i′, the aqueous diacetal f and high boiler fraction gand g′ which, in addition to other components, comprises the glyoxaldimethyl acetal and unconverted glyoxal will now be illustrated. In theworkup of the aqueous composition, the high boiler fraction may for themost part be recycled directly into the synthesis stage (stream g′)which not only gives a distinct improvement in yield but is also aparticularly environmentally friendly and resource-protective procedure.

The aqueous composition, stream d, is partly evaporated in apre-evaporator to obtain a biphasic stream e which is fed to thedividing wall column (TK).

The dividing wall column (TK) is divided by the dividing wall (T)disposed in the longitudinal direction into the subregions 1 to 6, i.e.into the upper common column region 1, the feed section havingrectifying section 2 and stripping section 4, the withdrawal sectionhaving rectifying section 5 and stripping section 3, and also the lowercommon column region 6.

The aqueous 1,1,2,2-tetramethoxyethane is removed as stream f as a sideeffluent (liquid or gaseous). The high boiler discharge or recycling iseffected via streams g and g′. The low boiler discharge or recycling iseffected via the streams i, i′.

It addition, water or aqueous glyoxal solution is introduced into thedividing wall column via stream m or k via the evaporator effluent.

Further definitions in FIG. 2 are: a methanol, b glyoxal solution, creactive feed, B1 stirred vessel to store the reactive mixture, B2vessel to store stream a and top effluent i′, P1, P2, P3 pumps withclosed-loop flow control, V evaporator, k evaporator effluent and hcondenser output.

IMPLEMENTATION EXAMPLE

Construction of the Laboratory Apparatus According to FIG. 2 forAcetalizing Glyoxal

A stirred vessel B1 having a capacity of about 0.5 l is initiallycharged with the reaction mixture and metered continuously (stream c)with a pump under closed-loop flow control (130g/h) into the reactorheated to 65° C. This tubular reactor consists of a jacket-heatedstainless steel tube of length 10 meters which has been charged with 620ml of catalyst (LEWATIT® K2629). The fine filters having a mesh width of140 μm which are mounted at the inlet and outlet of the reactor preventthe discharge of the catalyst. The effluent of this reactor d isconducted via a line into the feed point of the dividing wall column.

A pressure-retaining valve in this line ensures a constant operatingpressure in the reactor of about 0.5 bar above atmospheric pressure.

The dividing wall column is operated at a pressure of 500 mbar(absolute). The dividing wall column TK used is a glass laboratorycolumn of internal diameter 50 mm which is provided in the region of thedividing wall with a bed of 3 mm stainless steel mesh rings which isabout 80 cm in height. Above the dividing wall in the common columnregion 1, the column is equipped with 50 cm of a laboratory fabricpacking having a specific surface area of about 950 m²/m³ . Below thedividing wall (region 6), the column is provided with about 20 cm of thesame packing. The feed e was fed in liquid form into the middle of thedividing wall region, and pre-evaporation was dispensed with.

The liquid sidestream takeoff f in the withdrawal section is at the sameheight. Above the dividing wall, the effluent liquid is combined in acollector and divided between the feed and the withdrawal side of thedividing wall by a swiveling funnel mounted within the column. Thedivision ratio of liquid between feed and withdrawal side isadvantageously from 1:4 to 1:4.5. The withdrawal rate of the sidestreamis controlled via the column bottom fill level. An additionaltemperature control of the internal column temperature measured directlybelow the sidestream takeoff in the region 5 prevents, especially in thecase of non-steady-state operating states such as starting-up andrunning-down procedures, the discharge of off-spec side effluents.Typically, a temperature of around 80.5° C. is observed at the sideeffluent takeoff.

The internal column temperature in the upper half of the common columnregion 1 is set to between 47° C. and 49° C. by a closed-looptemperature-controlled reflux rate (stream i). The column is equippedwith a condenser K which is operated at about 5° C.

The column is equipped with a rotary film evaporator (4.6 dm²) which ischarged via a pump. Water and/or aqueous glyoxal solution (stream m) maybe metered into its feed under car control. The high boiler fraction g′is removed under closed-loop temperature control at about 92° C. in thecolumn bottom. Bottom and top effluent (streams g′ and i′ respectively)of the distillation column are recycled into the reservoir vessel B1.The entire column is equipped with adiabatic protective heaters.

EXAMPLE

418 g of a 40.7% glyoxal solution which contains, inter alia, 1% ofethylene glycol, and also 324 g of distilled water (stream m) aremetered within 24 hours into the bottom evaporator V of the dividingwall column. Bottom and top effluent (streams g′ and i′ respectively) ofthe distillation column are recycled into the reservoir vessel B1. Anamount of 30 g is discharged at the high boiler stream g within 24hours, which is necessary in order to restrict accumulation of highboilers. The 371 g amount of methanol consumed by the reaction ismetered under level control into the reservoir vessel B1 (stream a).Several days of continuous operation beforehand ensure steady-stateconditions in the laboratory apparatus. The effluents obtained areanalyzed by gas chromatography and the content of glyoxal is determinedtitrimetrically.

Under the above-described process conditions, 1045 g of an aqueous TMEsolution are obtained at the sidestream takeoff f of the dividing wallcolumn and contain, in addition to 37.6% by weight of1,1,2,2-tetramethoxyethane, traces of methanol and other secondarycomponents, also 0.15% of 2,3-dimethoxy-1,4-dioxane. This results inmolar yields of 89% of theory based on glyoxal and 91% based onmethanol.

Analysis of a sample of the feed stream e to the dividing wall columnreveals, in addition to methanol, among other components, 12.4% byweight of 1,1,2,2-tetramethoxyethane, 8.2% of water, 3.2% ofdimethoxydioxane and 1% of glyoxal. Analysis of a sample of the lowboiler effluent i′ reveals 99.5% by weight of methanol and, in additionto 0.2% of water, traces of other low boilers. In the discharged highboiler stream g, among other components, 26% by weight of glyoxaldimethyl acetal, 24% by weight of glyoxal, 20% by weight of water, 11.6%by weight of dimethoxydioxane and, in addition to traces of1,1,2,2-tetramethoxyethane, also other high boilers of which some areunknown.

In a further laboratory column which is to be operated continuously, isequipped with approx. 1.80 m of a laboratory fabric packing of specificsurface area 950 m²/m³ and has an internal diameter of 40 mm, thecollected side effluents of the dividing wall column are subjected to anazeotroping agent distillation at 500 mbar. This column too is equippedwith an oil-heated rotary film evaporator (4.6 dm²) and equipped withadiabatic protective heaters.

The feed is introduced in such a way that the stripping section of thiscolumn has a length of 80 cm.

Methylcyclohexane is used as an auxiliary. The water-containing vaporswithdrawn at the top of the column are introduced into a phase separatorafter condensation. The organic phase is fed back to the columns asreflux and the heavier aqueous phase is removed. The pure1,1,2,2-tetramethoxyethane is obtained at the bottom by closed-loop filllevel control. The purity of the product is ensured by closed-looptemperature control in the lower section of the column which acts on theevaporator heating. Typically, the bottom temperature is 135° C., while61° C. are measured at the top of the column. The effluents are analyzedby gas chromatography.

For example, 2921 g of 99.3% by weight pure 1,1,2,2-tetramethoxyethanewhich, in addition to 0.6% by weight of 2,3-dimethoxy-1,4-dioxane,contains only traces of water and other components are obtained within53 hours from 8032 g of an aqueous solution which contains 37.2% byweight of 1,1,2,2-tetramethoxyethane and 0.2% by weight ofdimethoxydioxane. In the aqueous top effluent, in addition to a smallamount of methanol (<0.1% by weight), traces of methylcyclohexane and220 ppm of glyoxal dimethyl acetal can be detected.

In a further apparatus which consists substantially of a heatablejacketed tube of length 4 m which is charged with 250 ml of catalyst(LEWATIT® K2629), a portion of the aqueous top effluent obtained aboveis further treated. Filters are again mounted at the inlet and outlet ofthis reactor, in order to prevent discharge of the catalyst.

62 ml/h of the aqueous discharge are continuously pumped through thecatalyst bed at ambient pressure. At an average reactor temperature of70° C., the glyoxal dimethyl acetal content could be depleted to valuesof <10 ppm, which corresponds to the limit of detection of the gaschromatography used.

1. A process for distillatively working up an aqueous composition whichcomprises 1,1,2,2-tetramethoxyethane, glyoxal dimethyl acetal andmethanol, wherein the composition is worked up to form at least one lowboiler fraction, at least one medium boiler fraction and at least onehigh boiler fraction in a dividing wall column in which a dividing wallis arranged in the longitudinal direction of the column to form an uppercommon column region, a lower common column region, a feed sectionhaving rectifying section and stripping section and a withdrawal sectionhaving rectifying section and stripping section, the dividing wall isarranged between the upper and the lower common column region and theaqueous composition is fed to the middle region of the feed section, andat least one medium boiler fraction comprising aqueous1,1,2,2-tetramethoxyathane is obtained as a sidestream from the middleregion of the withdrawal section, at least one high boiler fractioncomprising glyoxal dimethyl acetal from the lower common column regionand at least one low boiler fraction comprising methanol from the uppercommon column region.
 2. A process as claimed in claim 1, wherein theupper common subregion has from 5 to 50% of the total number oftheoretical plates of the dividing wall column, the rectifying sectionof the feed section has from 5 to 50% of the total number of theoreticalplates of the dividing wall column, the stripping section of the feedsection has from 5 to 50% of the total number of theoretical plates ofthe dividing wall column, the stripping section of the withdrawalsection has from 5 to 50% of the total number of theoretical plates ofthe dividing wall column, the rectifying section of the withdrawalsection has from 5 to 50% of the total number of theoretical plates ofthe dividing wall column, and the lower common section has from 5 to 50%of the total number of theoretical plates of the dividing wall column,the total number of theoretical dividing wall plates of the dividingwall column being 100%.
 3. A process as claimed in claim 1, wherein atleast one high boiler fraction is obtained as a liquid sidestream in thelower section of the column from 1 to 5 theoretical plates above thecolumn bottom and comprises glyoxal diniethyl acetal.
 4. A process asclaimed in claim 1, wherein glyoxal dimethyl acetal is isolated from atleast one high boiler takeoff of the dividing wall column.
 5. A processas claimed in claim 1, wherein the medium boiler fraction is withdrawnin liquid or gaseous form at the sidestream takeoff point and comprises1,1,2,2-tetramethoxyethane.
 6. A process as claimed in claim 1, whereinthe aqueous composition has one or more of the following contents whosesum does not exceed 100% by weight: from 8 to 28% by weight of1,1,2,2-tetramethoxyethane from 2 to 12% by weight of glyoxal dimethylacetal from 40 to 80% by weight of methanol from 5 to 18% by weight ofwater.
 7. A process as claimed in claim 1, wherein the aqueouscomposition is obtained by reacting from 40 to 75% by weight aqueousglyoxal with methanol in the presence of an acidic catalyst, by leavinga liquid mixture which, at the start of the reaction, contains methanoland glyoxal in a molar ratio of at least 15:1 and water in aconcentration of not more than 8% by weight in contact with the acidiccatalyst until the concentration of the 1,1,2,2-tetramethoxyethaneformed in the reaction mixture has reached at least 70% of theequilibrium concentration without more than 5% by weight of the methanolhaving been distilled off beforehand.
 8. A process as claimed In claim1, wherein the aqueous composition is obtained in the preparation ofglyoxal monoacetals of the general formula (I)

by, in the process, reacting a mixture of from 20 to 60% by weight ofaqueous glyoxal and glyoxal bisacetals of the general formula (II),

in the presence of an acidic catalyst with a excess of a monohydricalcohol ROH until the reaction equilibrium has been attained.
 9. Aprocess as claimed in claim 1, wherein the liquid or gaseous aqueousside effluent of the dividing wall column, which contains most of the1,1,2,2-tetramethoxyethane, is fed into a further column which isequipped with trays, structural packings or random packings and in whichthe water is removed with the aid of an azeotroping agent bydistillation.
 10. A process as claimed in claim 9, wherein the1,1,2,2-tetramethoxyethane is at least partly obtained as a liquid orgaseous sidestream in the lower section of the column, and/or the bottomtakeoff of this column is fed into the feed point of the dividing wallcolumn.
 11. A process for distillatively working up an aqueouscomposition which comprises 1,1,2,2-teiramethoxyethane, glyoxal dimethylacetal and methanol, which comprises carrying out the workup of thecomposition according to the process as defined in claim 1 in acombination of two distillation columns in the form of a thermalcoupling, which corresponds to the dividing wall column.
 12. A processas claimed in claim 2, wherein at least one high boiler fraction isobtained as a liquid sidestream in the lower section of the column from1 to 5 theoretical plates above the column bottom and comprises glyoxaldimethyl acetal.
 13. A process as claimed in claim 2, wherein glyoxaldimethyl acetal is isolated from at least one high boiler takeoff of thedividing wall column.
 14. A process as claimed in claim 3, whereinglyoxal dimethyl acetal is isolated from at least one high boilertakeoff of the dividing wall column.
 15. A process as claimed in claim2, wherein the medium boiler fraction is withdrawn in liquid or gaseousform at the sidestream takeoff point and comprises1,1,2,2-tetramethoxyethane.
 16. A process as claimed in claim 3, whereinthe medium boiler fraction is withdrawn in liquid or gaseous form at thesidestream takeoff point and comprises 1,1,2,2-tetramethoxyethane.
 17. Aprocess as claimed in claim 4, wherein the medium boiler fraction iswithdrawn in liquid or gaseous form at the sidestream takeoff point andcomprises 1,1,2,2-teiramethoxyethane.
 18. A process as claimed in claim2, wherein the aqueous composition has one or more of the followingcontents whose sum does not exceed 100% by weight: from 8 to 28% byweight of 1,1,2,2-tetramethoxyethane from 2 to 12% by weight of glyoxaldimethyl acetal from 40 to 80% by weight of methanol from 5 to 18%byweight of water.
 19. A process as claimed in claim 3, wherein theaqueous composition has one or more of the following contents whose sumdoes not exceed 100% by weight: from 8 to 28% by weight of1,1,2,2-tetramethoxyethane from 2 to 12% by weight of glyoxal dimethylacetal from 40 to 80% by weight of methanol from 5 to 18% by weight ofwater.
 20. A process as claimed in claim 4, wherein the aqueouscomposition has one or more of the following contents whose sum does notexceed 100% by weight: from 8 to 28% by weight of1,1,2,2-tetramethoxyethane from 2 to 12% by weight of glyoxal dimethylacetal from 40 to 80% by weight of methanol from 5 to 18% by weight ofwater.
 21. A process as claimed in claim 10, wherein the1,1,2,2-tetramethoxyethane is at least partly obtained as a liquid orgaseous sidestream in the lower section of the column from 1 to 5theoretical plates above the column bottom.